18.42 So a slightly disappointing end to the night for Britain but it's not going to take the gloss off what has been a truly historic few hours. Adam Gemili kicked things off by becoming only the second Briton to run a sub-20sec 200m and then it was over to Mo Farah. The double Olympic champion has become the double world champion. I'm going to stick my neck on the line: he is now the greatest British athlete of all time. How lucky we all are to watch him in action and have the chance to debate such things. That will be all from me. Thanks for joining me. Cheerio.

18.38 The British team were in a great position after two legs but Bingham and Levine failed to do the job. Jamaica and Russia seized the chance, pulled away and Levine couldn't get close after getting trapped in. If you're wondering where the Americans were, they were running a different race 10m in front of everyone else. Britain have actually run a slower time than they did in the heats.

18.35 Levine on the final leg for Britain but he's got trapped in. He's in a terrible position. USA take gold, Jamaica take second, Russia third and Britain have to make do with fourth. That was not a very good final leg.

18.34 There are so many of them in with a chance of a medal here. USA out in front. Belgium now second and Britain remain third with Jamaica and Russia just behind. One man to go.

18.33 First handover and now they reach the break. USA lead, Russia are second with Rooney tucking in third for Britain.

18.32 Off they go and Williams leads out the British team.

18.30 Just the one event left then. It's relay time and how about this to get you in mood. Kriss Akabusi you hero!

18.26 Not a whole lot of sympathy for Felix from former British sprinter Darren Campbell!

18.24 What a shame for Felix. It looked like some sort of ankle or hamstring injury and with that, everyone's hopes of an epic head-to-head battle were over. Ahoure gets her second silver medal of the championships and Okagbare takes bronze. And the slow-motion replay shows that Fraser-Pryce did it all with her laces undone!

18.22 Bang! Oh no, Felix has gone. She collapses to the floor on the bend with an injury and away goes Fraser-Pryce. The two Africans are chasing her but they can't get near and Shelly-Ann Fraser-Pryce is the double 100m and 200m champion!

18.19 I haven't just ignored the women's 200m final. They have delayed it slightly. But the athletes are all lined up now. Will it be a straight shootout between Shelly-Ann Fraser-Pryce and Allyson Felix? Or can Murielle Ahoure or Blessing Okagbare spring a surprise? Let's find out.

18.16 I do understand what Trudy Wren is saying... but I'd rather play the "who is greater" game:

Is Mo Farah the best athlete in the world if he scores a double double? No, he is definitely comparable, but not the greatest

Look at the stars like Seb Coe, Steve Ovett, Steve Cram, Daley Thompson and all other sport stars who have made history or broken records, ALL great in their own way. ALL unique for those records and medals they have to their names

No one person is the greater or the greatest - all are the Greatest. Ask Mr Ali

18.15 Before we move on let's just savour this image.

18.09 Well that was the BIG ONE, but we are going to stay with you for the two remaining track events because they are tasty as well. First up is the women's 200m final at 6.15pm before Britain aim for a medal in the men's 4x400m relay 15 minutes later.

18.07 Just to clear up the medallists. Gold goes to... Farah. Gebrhiwet came through for sIlver and Koech took bronze.

18.03 Mo Farah is the Olympic 5,000m champion, the Olympic 10,000m champion, the world 5,000m champion and the world 10,000m champion. Wow. If he's not the greatest ever then he's pretty darn close.

18.01 We know what Farah loves to do. He hits the front about 600m out and simply does not let anyone get in front of him. And he executed his plan to perfection.

17.58 Farah still leads. Can he hold them all off? Into the home straight. Here come the Kenyans. Can Koech pass? Farah kicks again. FARAH IS THE 5,000M WORLD CHAMPION!!!

17.57 / 1 laps to go The race has begun. And there goes Farah. He ups the pace and takes the lead. This is when Mo says: "You shall not pass".

17.56 / 2 laps to go This is going to be a very fast finish. It's anyone's race at the moment as Alamirew and Farah head the pack with the three Kenyans right in behind them.

17.54 / 4 laps to go This is very, very slow. Koech goes back to the front with Farah second. He won't want to get any further back now. In fact Koech is just starting to wind up the pace ever so slightly. And still we wait...

17.52 / 6 laps to go Here he comes. Farah moves to the front to remind his rivals of his presence, while slowing things down further. They are all playing into Farah's hands at the moment. It's exactly as the Briton would have wanted it so far.

17.50 / 8 laps to go Well that little break didn't last long and the Kenyans are back with the pack now. If anything that move will have tired Koech and Longosiwa out more than anyone else. It's all a bit sedate and Ethiopian Muktar Edris is at the front with Farah happy to sit near the back.

17.48 / 10 laps to go This is interesting. Koech and Longosiwa, two of the three Kenyans have gone out ahead and have a decent lead at the moment. Farah isn't bothered though and is about 30m behind in 10th place.

17.46 12 and a half laps ahead of them and off they go. Farah settles into third place with Lagat leading the way and setting a slow pace.

17.34 That's all of the pre-race build-up done. It's Farah time. Mo is undoubtedly the class act of the race but he is also one of only two runners in the 15-man final who ran in the 10,000m last weekend. Only time will tell how much that has taken out of him.

17.30 It's not to be for Okoro either I'm afraid. The slow first lap didn't suit the British runner and she didn't have a big enough kick to get inolved at the end, eventually finishing 7th as well.

17.22 Cracking finish to the race but Muir wasn't involved and could only manage to finish 7th. Let's see if Marilyn Okoro can do any better in the second semi.

17.19 We move swiftly onto the women's 800m semi-finals and Laura Muir goes for Britain in the first one (there are only two semis in this event).

17.18 The stadium has just erupted and the reason for that is Tatyana Lysenko. The Russian has taken gold in the women's hammer with a new championship record of 78.80m. Loving this tweet from John Regis:

&lt;noframe&gt;Twitter: John Regis - Young Adam Gemeli is trying to break my british record!!!! Great run 19.98.. at last I have a COLLEAGUE in the sub 20 club!!&lt;/noframe&gt;

17.08Lindsay Campbell-Peat has got in touch about the Farah greatness debate. And she reckons gold will make him the best British athlete ever:

Definitely yes. He’s an amazing athlete – being able to compete consistently at such a high level in both the 5,000m and 10,000m shows how good he is – he is in the form of his life right now and can still get better. He’s also such a great personality, in the last couple of years people have warmed to him like no other and he’s managed to get young people excited about running and athletics which is positive for the future. I’m nervous though – Soi is a threat – but conditions are good so fingers crossed! GO MO!

17.04 Wow! That really was incredible. John Regis is the only Briton to have EVER run the 200m faster than Gemili. Here is confirmation of what the 19-year-old has just achieved. Yep, he's still only 19.

17.01 Here are some words from Gemili: "I can't believe it. That is the best feeling in the world. I'm over the moon. I've made the final and run a PB and I'm so grateful."

16.59 Unbelievable scenes! Gemili wins the race in 19.98secs and becomes only the second Briton ever to dip under the 20sec barrier. That is serious sprinting. Serious sprinting.

16.56 Two down, one to go. And it's Britain's best chance of getting someone through to the 200m final with Adam Gemili, who ran a personal best 20.17secs in the heats this morning. He is up against four sub-20secs runners though.

16.50 Guess who won? Yep, you're right. Although amusingly Bolt almost eased down too much in the closing stages and had to put his foot on the gas slightly with 10m to go. Victory for the 100m champion but James Ellington could only manage fourth and will not be in tomorrow's final.

16.47 Recognise this guy? It's his turn now and Britain's James Ellington is two lanes outside him.

16.43 Hmmm... everyone expected the London 2012 bronze medallist Warren Weir to walk through to the final but he was made to work hard and could only finish second in that semi. The unlikely winner was Curtis Mitchell of the USA who blitzed away from the field. And he is P.U.M.P.E.D. Williams was left a long way back and could only manage 7th place.

16.40 The first of three (yep, I know it doesn't make sense) semi-finals is about to begin and Britain have Delano Williams in it. He is in the outside lane so has a tough task ahead of him.

16.38 The 200m semi-finals are about to begin but there is someone missing from these championships through injury. He goes by the nickname The Beast, but is more commonly known as Yohan Blake. He has just been speaking to the BBC:

Tears were in my eyes watching the 100m final but Usain Bolt did really well. I'm 95 per cent right now. I just wasn't going to be fit enough for these championships so I decided to call it a day for this year.I'm looking to the future because there are good days to come.

16.30 Let's get back to the Farah greatness debate. It appears some people have a problem with calling Farah the "greatest ever" when he is still at a relatively early stage of his career. But then few people would argue that Usain Bolt is not the finest sprinter the world has ever seen.

So if not Farah (2 x Olympic gold, 2 x world gold, 1 x world silver) then who is Britain's greatest ever? Here are some options:

16.11 Well done that man! O'Hare held the inside line, waited for a gap to open and was strong enough to seize the advantage to finish a brilliant fourth. Not bad considering his PB was slower than all but one of his rivals and O'Hare will now line up in the 1,500m final on Sunday.

16.06 The men are underway in the first semi-final of the 1,500m and Chris O'Hare is Britain's sole representative. He's up against it though with 10 of his competitors having previously run faster than his personal best.

16.02 The first track action of the afternoon is just couple of minutes away so before then, why not check out this short Farah interview after he won the Rock 'n Roll half marathon in New Orleans earlier this year. Cringe doesn't even come close. "Haven't you run before...?"

Paula Radcliffe retains this comical vision of seeing the young, dreadlocked teenager Mo Farah in Vilamoura, when the British athletes were sharing the same hotel as a bunch of old ladies on a Saga holiday.

“He kept jumping into the pool and splashing and they were all complaining, ‘ooo, won’t you control that naughty little boy?’” recalls Farah’s old mentor with a smile here in Moscow.

“So whenever he gets a bit too big for his boots, I just tell him, ‘To me, you’ll always be that naughty little boy.’”

The incredulous thing, laughs Radcliffe, is that the naughty boy could by Friday night complete his graduation to being the greatest British athlete of all time.

15.42 As the clock ticks down towards show time, we have plenty of Farah and other World Championships pieces on offer for you.

15.25 How do you define greatness? I'lll tell you: you can't. Greatness is subjective. What one person may deem the greatest feat the world has ever seen, another might think, "yeah, that was alright I suppose".

One thing is clear though: Mo Farah is not just "alright I suppose". He will undoubtedly go down as one of the legends of athletics. Just to recap, the Somali-born, Oregon-based, British athlete currently holds both the 5,000m and 10,000m Olympic titles, as well as the world 10,000m title he won last Saturday. If he takes 5,000m gold this afternoon he will become only the second man ever to hold the Olympic and world distance double at the same time after Ethiopian Kenenisa Bekele.

So we're all happy that Farah is more than a little bit good? Right. Let's head back to the question of greatness and remember, there is no wrong answer here. Will Mo Farah become Britain's greatest ever athlete if he wins gold today?

You see, no one knows the answer. But it won't stop us leading a live blog on the subject...

The 5,000m starting gun will go off at 5.45pm but there is plenty of action before that, including some bloke called Usain Bolt running in the men's 200m semi-finals, which also feature a triple whammy of Britons in Adam Gemili, Delano Williams and James Ellington. We'll also have news from the men's long jump final as well as Laura Muir and Marilyn Okoro's quest in the women's 800m semis. Lot's of fun to come.